1. Field of the Invention
The present invention relates to a displacement measuring method for measuring a minute displacement of a probe in a quantity on the order of a nanometer or smaller which has resulted from a change in atomic force acting between the probe and a sample. More specifically, the present invention relates to a displacement measuring method suited for the detection of a surface profile of or surface indents appearing on a surface of an article such as, for example, an integrated circuit board.
2. Description of the Prior Art
The prior art minute displacement measuring method utilized in an AFM (atomic force microscope) system for the measurement of the minute surface profile of a sample comprises a detection of a deformation of a probe mounted on a cantilevered elastic beam and disposed in the close vicinity of a surface of the sample to be measured. The deformation of the probe is a function of the atomic force acting between the probe and the surface of the sample to be measured. Then, by varying the position of the sample in a transverse direction perpendicular to the probe while the atomic force so detected is maintained at a predetermined value, the surface profile of the sample is measured by moving the surface of the sample along a plane perpendicular to said transverse direction while the spacing between the to-be-measured surface and the probe is precisely kept at a predetermined distance on the order of a nanometer to a subangstrom.
The prior art minute displacement measuring method referred to above will now be discussed in detail with reference to FIG. 5. The system shown therein includes a laser source 1 designed to emit linearly polarized beams P and S having respective P- and S-polarizing directions inclined at an angle of 45 degrees relative to a plane of the sheet of FIG. 5. These linearly polarized beams travel through a beam splitter 3 towards a Wallaston's prism 4. The rays of light incident on the Wallaston's prism 4 are then separated in two directions corresponding to the P- and S-polarizing directions. In the illustrated example, the linearly polarized beam P in the P-polarizing direction is separated upwards as a measuring beam P while the linearly polarized beam S in the S-polarizing direction is separated downwards as a measuring beam S.
The measuring beam P is, as it pass through a lens 5, converged at point adjacent the tip of the displacement measuring probe 6 and is then reflected backwards so as to travel again towards the Wallaston's prism 4 through the lens 5. The reflected measuring beam P reflected from the probe 6 and having subsequently passed through the prism 4 again enters the beam splitter 3 and is subsequently deflected so as to travel towards a prism 8. This prism 8 is either a light transmissive type or a reflecting type depending on the direction of polarization of the light.
The measuring beam P emerging from the prism 8 enters a photodetector 9 by which the focal position of the measuring beam P is detected. In FIG. 5, the measuring beam P shown by the solid line represents that before a displacement of the probe 6 and the measuring beam P shown by the phantom line represents that after the displacement of the probe 6. Therefore, if the focal position of the measuring beam P is displaced upwards as viewed in FIG. 5, an output signal from an upper terminal of the photodetector 9 indicative of the focal position will be of a level higher than an output from a lower terminal of the photodetector 9, and vice versa.
The output signal indicative of the focal position of the measuring beam P outputted from the photodetector 9 is then processed by a signal processing circuit, including a subtractor 11, an adder 13 and a divider 15, so as not to be affected by a change in signal intensity of the light source and is then converted into an electrical signal.
On the other hand, the measuring beam S is converged by the lens 5 at a point adjacent a probe holder 7 for the support of the probe 6 and is subsequently reflected therefrom. The reflected measuring beam S is then passed through lens 5 so as to travel towards the prism 4. The measuring beam S deflected by the prism 4 so as to enter the beam splitter 3 by which the measuring beam S is reflected so as to travel towards the prism 8. As the measuring beam S enters the prism 8, the measuring beam S is allowed to pass through or reflected by the prism 8 depending on the direction of polarization as is the case with the measuring beam P described above.
The measuring beam S emerging from the prism 8 is subsequently detected by the photodetector 10 to determine the focal position of the measuring beam S in a manner similar to the detection of the focal position of the measuring beam P described above.
The output signal indicative of the focal position of the measuring beam S outputted from the photodetector 10 is then processed by a signal processing circuit, including a subtractor 12, an adder 14 and a divider 16, so as not to be affected by a change in signal intensity of the light source and is then converted into an electrical signal.
In the foregoing prior art method, the photodetector 9 provides a composite output comprises of a signal indicative of a change in angle resulting from the displacement of the probe 6, a noise signal resulting from a swaying motion of air and a noise signal resulting from a vibration of the optical system. On the other hand, the photodetector 10 provides a composite output comprises of a noise signal resulting from a swaying motion of air and a noise signal resulting from a vibration of the optical system.
Accordingly, if the composite signal from the photodetector 10 is subtracted by a subtractor 17 from the composite signal from the photodetector 9, it is clear that the signal substantially faithfully indicative of the minute displacement of the probe 6, which is not affected by the swaying motion of air and the vibration in the optical system, can be obtained.
Thus, by varying the position of the sample 18 in a transverse direction perpendicular to the probe 6 by means of an XYZ scanner 19 supporting the sample 18 while the displacement signal so obtained is maintained at a predetermined constant value, the surface profile of the sample 18 can be measured by moving the surface of the sample 18 along a plane perpendicular to said transverse direction while the spacing between the sample 18 and the probe 6 is kept at a distance on the order of a nanometer to a subangstrom.
In the prior art method, however, it is not possible to converge and focus the measuring beams P and S at the probe 6 and the photodetectors 9 and 10 simultaneously and, therefore, when the measuring beams P and S are converged and focused on the probe 6, the measuring beams P and S cannot be converged and focused on the photodetectors and, therefore, the detecting sensitivity to the displacement of the probe is lowered. On the other hand, when the measuring beams are focused on the photodetectors, the measuring beams cannot be focused on the probe 6 and, therefore, where the probe 6 is in the form of a minute probe formed by the use of a process such as, for example, a semiconductor lithography, the spot size of the measuring beams becomes larger than the width of the minute probe with a portion of the light falling on the sample, which portion is, after having been scattered from the sample, converged again by the lens 5 so as to fall on the photodetectors 9 and 10 with the consequence that this signal eventually overlaps a normal signal as a ghost signal. Therefore, a signal indicating as if the probe 6 is located at a position different from the position to which it has actually been displaced is outputted, resulting in an error in measurement.